Hi guys,
I've been self educating myself on CNC milling for the last year or so. Up to now I have milled mainly Plywood and MDF. Started experimenting with aluminum just recently. Today I started my first big cut. This one should take somewhere about 70+ hours. Started my first operation, that should last for about 7:30 hours. At exactly 2 hours and 10 mins the endmill broke. I believe I used moderately light feedrate, stepover and depth, so I am curious to find out what caused the breakage.
There 3 possible reasons that I can think of:
1. Overheating of both the stock and the endmill
2. ****ty chinese endmills
3. I am mistaken and I need to further reduce the feedrate and stepover.
So here is some info on the machine and settings I am using.
CNC router equipped with a 2.2 kW ER20 chinese spindle. Max feedrate 1200 mm/min min/max RPM 3000/21800. Since those spindles have very low torque at low rpm, I prefer faster cuts with higher rpm.
The cut is done with a 6mm, 2 flute, flat HSS endmill (from Aliexpress), which is sticking out 33mm. Feedrate is 750 mm/min, rpm ~15100. I have equipped DIY mist cooling. It is powered by an 8bar compressor (limited to 2.5-3 bar for this operation). For coolant I use hydraulic oil (VG32).
The stock I am working on is 6082 T6 aluminum slab with dimensions 500x330x40 mm. This first cut goes down 19mm deep, at 2mm stepdown increments, and 1.8mm stepover.
I have came up with those settings using GWizzard. Any more feedrate/stepdown/stepover increases the tool deflection to "red" numbers. For those 2 hours everything seemed and sounded fine, until suddenly the endmill broke. Examining the broken piece I didn;t notice any wear on the cutter, so that shouldn;t be the problem.
One thing that I noticed. About 10 minutes after the endmill broke, I touched the stock it was roughly 50 degrees celsius to the touch. Which means it was even hotter at the time of breakage, and at the place of the cut the temp was probably even higher (maybe well over 100 degress). Could the temp of the stock have affected the properties of the aluminum?
If this is the case, maybe I should spray some water on the stock from time to time, hoping that evaporation will cool it. And maybe a short break after each hour of milling? I'm even thinking about switching the oil with water, but I am not sure how good of an idea that is.
I need to improve the rate of breakage. I have at least 10 more hours of milling with a 6mm endmill, and I have about 5 more such endmills. If these rates keep up I might need more endmills
Attached you can see my G-Wizzard screen, and the piece of stock right after the breakage.
Edit: Forgot to mention. This was a brand new endmill, so those were the first 2 hours and 10 minutes of its usage.
And one more question. See those circles on the stock? Those are eventually going to be the mounting place for a COB LED, I need them perfectly flat. But after the current operation they are not perfectly flat. They are actually perfectly flat almost all over, except for a small notch at the exact center of the circle. It is obvious I need to do a second operation to fix this, but if I use the same cutting strategy, I'll probably get the same notch. I am thinking about using parallel passes instead of a spiral. Sure that might leave some bumps near the circle's boundary, but the center part should be fine, and that is what matter to me.
Would love to hear your thought on that.